In a Long Term Evolution-Advance (LTE-A) system, in order to reduce the adjacent cell interference of User Equipment (UE) at a cell coverage edge and improve the experience of the user equipment at the cell edge, a coordinated multipoint transmission technology is adopted. The coordinated multipoint transmission technology is cooperation among multiple Transmission Points (TPs) separated geographically. In general, the multiple transmission points refer to base stations of different cells or the base station of one cell and multiple Remote Radio Heads (RRHs) controlled by the base station. The coordinated multipoint transmission technology includes downlink coordinated transmission and uplink joint reception. The downlink coordinated multipoint transmission mainly includes two transmission solutions: Coordinated Scheduling/Coordinated Beamforming (CS/CB) and Joint Processing (JP). In CS/CB transmission, one transmission point among the multiple transmission points sends a useful signal to the user equipment, and the other transmission points reduce the interference on the user equipment as much as possible by joint scheduling and beamforming. The joint processing solution includes Joint Transmission (JT) and Dynamic Point Selection (DPS). In JT transmission, the multiple transmission points simultaneously send the useful signal to the user equipment, thus enhancing the receiving signal of the user equipment. In DPS transmission, the transmission points of the user equipment are dynamically switched, and the transmission point optimal to the user equipment is always selected from the coordinated transmission points to transmit a user equipment signal. These coordinated multipoint transmission solutions may be combined for use. A Dynamic Blanking (DB) solution may also be combined to dynamically set certain transmission points to send no signal on certain time-frequency resources.
In order to support coordinated multipoint transmission, the user equipment needs to report the Channel State Information (CSI) of multiple transmission points. A network configures the report of the multipoint channel state information of the user equipment by configuring multiple CSI processes for the user equipment. One CSI process is a combination of a Non-Zero Power (NZP) Channel State Information Reference Signal (referred to as CSI reference signal or CSI-RS for short) resource used for measuring channel information and an Interference Measurement Resource (IMR) used for measuring interference.
The definition of an existing CSI reference resource is as follows:
firstly, on a frequency domain, the CSI reference resource is defined as a group of downlink Physical Resource Blocks (PRBs) corresponding to a frequency band related to a Channel Quality Indicator (CQI) reported by the user equipment; for example, if the CQI reported by the user equipment is a wideband CQI, the frequency domain resource of the CSI reference resource is all PRBs on the entire system bandwidth, if the CQI reported by the user equipment is a sub-band CQI, the frequency domain resource of the CSI reference resource is all PRBs on the sub-band;
secondly, one a time domain, the CSI reference resource is defined as a single downlink subframe n−nCQI_ref, wherein:
n corresponds to a CSI information reporting subframe, for periodic CSI report, nCQI_ref is a minimum value larger than or equal to 4, and the n−nCQI_ref corresponds to an effective downlink subframe;
for aperiodic CSI report, nCQI_ref is a value such that the CSI reference resource and a CSI request, in an uplink Downlink Control Information (DCI) format triggering the aperiodic CSI report, in the same subframe, and the n−nCQI_ref corresponds to an effective downlink subframe;
for the aperiodic CSI report, nCQI_ref is equal to 4, the downlink subframe n−nCQI_ref is an effective downlink subframe, and the user equipment receives the subframe after receiving the CSI request in random access request permission.
The conditions that the downlink subframe in the above-mentioned serving cell is considered as the effective downlink subframe are as follows:
the downlink subframe is a downlink subframe configured for the user equipment; and
except a transmission mode 9, the downlink subframe is not a Multimedia Broadcast multicast service Single Frequency Network (MBSFN) subframe; and
if the Downlink Pilot Time Slot (DwPTS) length of a particular subframe is 7680·Ts or shorter, the particular subframe is not an effective downlink subframe; and
the downlink subframe is not in a measurement gap configured for the user equipment; and
for the periodic CSI report, if the user equipment is configured with a CSI subframe set, the downlink subframe is an element in a subframe set related to the periodic CSI report.
If there is no effective downlink subframe for the CSI reference resource in some serving cell, the CSI report of the serving cell in an uplink subframe n is discarded.
Thirdly, on a layer domain, the CSI reference resource is defined by Rank Indication (RI) corresponding to the CQI and a Pre-coding Matrix Indicator (PMI).
The frame structure type of LTE/LTE-A includes a frame structure type 1 and a frame structure type 2. The frame structure type 1 is applied to full duplex and half-duplex Frequency Division Duplex (FDD). The length of each radio frame is Tf=307200·Ts=10 ms, 20 time slots are included, the length of each time slot is Tslot=15360·Ts=0.5 ms, and the time slots are sequentially numbered from 0 to 19. Each subframe includes two continuous time slots 2i and 2i+1. For FDD, within each period of 10 ms, 10 subframes may be applied to downlink transmission, and 10 subframes may be applied to uplink transmission. The uplink and downlink are separated on the frequency domain. FIG. 1 shows a schematic diagram of the frame structure type 1.
The frame structure type 2 is applied to Time Division Duplex (TDD). The length of each radio frame is Tf=307200·Ts=10 ms, 2 half frames are included, and the length of each half frame is 153600·Ts=5 ms. Each half frame includes 5 subframes with lengths of 30720·Ts=1 ms. The frame structure type 2 supports 6 uplink-downlink configurations, as shown in the following table 1, for each subframe in the radio frame, D represents the subframe is used for downlink transmission, and U represents the subframe is used for uplink transmission, and S represents a particular subframe. One particular subframe includes 3 domains: a DwPTS domain, a Guard Period (GP) domain and an Uplink Pilot Time Slot (UpPTS) domain. Each subframe includes 2 time slots, 2i and 2i+1, and the length of each time slot is: Tslot=15360·Ts=0.5 ms. FIG. 2 shows a schematic diagram of the frame structure type 2.
TABLE 1Downlink-to-UplinkUplink-Switch-downlinkpointSubframe numberconfigurationperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
Compared with the traditional single-cell transmission, in the coordinated multipoint transmission, the user equipment needs to report more feedback information. For periodic report, in one carrier, the user equipment needs to perform multiple groups of periodic report; for aperiodic report, the user equipment reports more information upon one trigger. Therefore, the user equipment needs to spend more resources and time to measure and calculate the reported information. Compared with the single-cell transmission, in the coordinated multipoint transmission, in a single carrier, the design that one CSI feedback corresponds to a CSI reference resource will obviously increase the complexity requirements of measurement and calculation of the user equipment.